1. Field of the Invention
The present invention relates to a strain or stress gauge and a method for detecting strain or stress of a structure using the gauge, and a plastic composite material including a bundle of electrically conductive fibers which has a breakdown foreknowledge mechanism and a method for foreknowing the progress of breakdown of a structure using the same.
Especially, the present invention relates to a strain or stress gauge and a method for detecting strain or stress of a structure using the gauge, which can detect the state or the history of strain or stress of the structure, or which can judge for itself to warn the progress of breakdown of the material or the like, on the basis of the electric resistance value or the changing state thereof in the bundle of the electrically conductive fibers.
2. Related Art
A method using a resistance wire strain gauge has been known as a means for detecting stresses generated in a structural member. The resistance wire strain gauge, as well known, has a meandering -like resistance wire made of platinum or the like. The strain gauge is attached on an external surface of an object to measure the stress thereof. The strain of the object is measured by the resistance change on the basis of the shape change of the resistance wire, i.e., the change of length and-cross sectional area of the resistance wire. The stress is calculated according to the measured strain.
However, according to such a conventional method, there are problems that the strain gauge is expensive and is limited in size. In the strain gauge, there is a linear relationship between load and strain. Therefore, even if a large live load is temporarily applied to the strain gauge, the display indicates zero strain after the live load is removed. If the measurement timing is missed, although the live load in a dangerously high range had been applied to the object and an excessive stress had been produced in the object in fact, a serious problem arose that this fact was not detected by the strain gauge. Furthermore, since the strain gauge has to be provided on the external surface of the object, there is a problem that an accurate internal stress cannot be detected when the object is large.
A FRP (Fiber Reinforced Plastic) composite material, which comprises plastics containing fibers, has been known. The FRP composite material has been used in many industrial fields because of an excellent specific tensile strength. In particular, the composite material has been recently used as a material having not only an excellent specific tensile strength but also a high durability and resistance to environmental factor's, in architecture and civil engineering fields.
However, since the FRP composite material is manufactured by incorporating reinforcing fibers into a matrix (a basic material comprising resin), the properties of the composite material tend to lack of uniformity.
Therefore, it is difficult to obtain a quantitative proof for backing up the strength of the FRP composite material. In this sense, such a conventional composite material is short complete reliability.